1. Field of the Invention
The present invention relates to a control system for a torque converter, and in particular, to a control system for a torque converter with a lockup clutch directly connecting an input shaft with output shaft thereof.
2. Description of Related Art
There is known a conventional control system for a torque converter with a lockup clutch. Japanese Patent Laid-Open Publication No. 1-250667 discloses such a conventional control system for a torque converter with three valves, including a lockup shift valve selectively supplying fluid to an engaging side fluid chamber and a releasing side fluid chamber, respectively disposed in left and right portions, from the lockup clutch, a lockup control valve controlling the pressure difference between the fluid pressures in the engaging side fluid chamber and the releasing side fluid chamber, and a lockup timing valve controlling the lockup clutch to be in a complete engaging condition. The system further includes a duty solenoid valve for controlling the lockup control valve and one ON-OFF solenoid. The two valves of the lockup shift valve and the lockup timing valve are controlled based on both the control pressure generated by an OFF (closed) operation of the ON-OFF solenoid valve and the duty control pressure generated by the duty solenoid valve.
Table 1 shows conditions of the three valves, such as the lockup shift valve, the lockup control valve and the lockup timing valve, corresponding to conditions of the two solenoid valves and of the lockup clutch.
TABLE 1 __________________________________________________________________________ ON-OFF DUTY LOCKUP LOCKUP LOCKUP SOLENOID SOLENOID SHIFT TIMING CONTROL CONTROL VALVE VALVE VALVE VALVE VALVE CONDITION __________________________________________________________________________ CONDITION A ON HIGH DUTY RELEASING ENGAGING ENGAGING STRONG SLIP (OPENED) RATIO PRESSURE FORCE FORCE CONDITION DRAINED CONTROL CONTROL POSITION POSITION POSITION CONDITION B ON LOW DUTY DRAINED ENGAGING ENGAGING WEAK SLIP (OPENED) RATIO POSITION FORCE FORCE CONDITION CONTROL CONTROL POSITION POSITION CONDITION C OFF ON (100%) DRAINED COMPLETE LOCKUP COMPLETE LOCKUP (CLOSED) POSITION LOCKUP POSITION CONDITION CONTROL POSITION CONDITION D OFF OFF (0%) INTRODUCING ENGAGING NON-LOCKUP CONVERTER (CLOSED) POSITION FORCE POSITION CONDITION CONTROL POSITION __________________________________________________________________________
However, since the two solenoid valves control the operations of both of the lockup shift valve and the lockup timing valve, the conventional control system has problems such as those described below.
Referring to Table 1, such problems occur when engaging force control conditions or conditions A and B are respectively changed to a complete lockup condition C or a converter condition D. Referring to FIG. 5, the ON-OFF solenoid can change from an ON condition to an OFF condition quickly. On the other hand, the duty solenoid valve needs to change a present value of the duty ratio, shown as X, to 100% or 0%. As a result, since the complete lockup condition or the converter condition is not established while the duty ratio is changing from X to 100% or from X to 0%, a responsiveness of the lockup clutch is decreased.
In order to resolve this problem, the duty solenoid valve could be is controlled based on the characteristics provided by an OFF operation of the ON-OFF solenoid valve which is shown as a dashed line in FIG. 4. This control also has problems when the converter condition is necessarily established, even when the complete lockup condition need to be established when the duty ratio X of the duty solenoid valve is located to the left of a switching point D. On the contrary, the perfect lockup condition is necessarily established, even when the converter condition needs to be established, when the duty ratio X is located to the right side of the switching point D.
Specifically, the converter condition needs to be established quickly so as to prevent engine stalling when the driver depresses the brake pedal rapidly under the condition B or the engaging force control condition. However, in this situation, there occurs a problem in that the complete lockup condition is established by the lockup timing valve being changed to the complete lockup control position as shown in Table 1 when the ON-OFF solenoid valve is changed directly to the OFF condition. In order to prevent this problem, the ON-OFF solenoid valve must be changed from the ON condition to the OFF condition after the duty ratio of the duty solenoid valve is once decreased to 0%. As a result, the operation of changing to the converter condition needs more time and, therefore, engine stalling may easily occur.
On the other hand, Japanese Laid-Open Patent Publication No. 63-186055 discloses regulating pressure valves and switching valves employed in a hydraulic circuit in an automatic transmission.